1. Field of the Invention
The present invention relates to a surface acoustic wave device such as a surface acoustic wave resonator, a surface acoustic wave filter, a sharing device, or other suitable device, and more particularly, to a surface acoustic wave device which uses a Shear Horizontal wave (“SH wave”).
2. Description of the Related Art
Conventionally, surface acoustic wave devices have been widely used as band-pass filters for use in mobile communication equipment. One such conventional surface acoustic wave devices include a surface acoustic wave resonator having an IDT (interdigital transducer) composed of interdigital electrodes having electrode fingers interdigitated with each other, the IDT being disposed on a piezoelectric substrate, and a surface acoustic wave filter using the surface acoustic wave resonator.
In such a surface acoustic wave device, a technique is known in which a leaky surface acoustic wave having a large attenuation which propagates in a Y-X LiTaO3 substrate with Euler angles (0°, −90°, 0°) as a piezoelectric substrate is converted to a Love wave type surface acoustic wave having no propagation loss by providing an IDT having a predetermined thickness and made of a metal having a large mass load such as Au, Ta, W, or other suitable metal.
FIG. 11 is a graph showing the variation of the electromechanical coupling coefficient k with the film thickness H/λ of Au electrodes (electrode film thickness/wavelength of excited surface acoustic wave), when the Au electrodes are provided on an LiTaO3 substrate of Y cut X propagation type, that is, having Euler angles of (0°, −90°, 0°).
As shown in FIG. 11, a leaky surface acoustic wave is produced when the film thickness H/λ of the Au electrodes 0.03 or less. In the range of H/λ of at least 0.004, a Love wave is produced. FIG. 12 is a characteristic graph showing the propagation loss (attenuation constant) of the leaky surface acoustic wave under the same conditions as those of FIG. 11. The solid line represents the propagation loss when the electrodes are in the electrical short-circuiting state, and the dotted line represents the propagation loss when the electrodes are in the open-circuiting state. As shown in FIG. 12, in the electrical short-circuiting state, the propagation loss is zero in the range of H/λ of about at least 0.033, and in the electrical open-circuiting state, the propagation loss is zero in the range of H/λ of about at least 0.044. Accordingly, to use an SH type surface acoustic wave having no propagation loss, the thickness H/λ of the Au electrodes in the electrical short-circuiting state is required to be at least 0.033, depending on the metalization ratio of the IDT. Further, for material such as Ta, W or other suitable material having a lower density than Au, the thickness H/λ must be more than 0.033.
However, as the thickness of IDT increases, the production accuracy decreases. Accordingly, a sufficiently large thickness cannot be achieved. Unless the film thickness is adequately large, for example, the thickness H/λ is at least 0.033 for the Au electrodes, a propagation loss of zero cannot be achieved.
On the other hand, the film thickness H/λ (electrode thickness/excited SH wavelength) at which the electrode fingers of an IDT can be made with general accuracy is up to 0.05.When the propagation loss is desired to be zero, the film thickness H/λ is required to be at least 0.033. Thus, the range of the film thickness where the electrode fingers of IDT can be formed with high accuracy is very narrow.
Further, if an IDT is formed of an electrode material having a slightly lower density than Au, such as Ta or W, the thickness of the electrodes must be further increased as compared with that of the Au electrodes. Thus, the propagation loss cannot be reduced to zero in the range of film thickness in which the film can be accurately formed.
Regarding materials such as Au having a considerably higher density as compared with electrodes materials generally used in the IDTs of surface acoustic wave devices such as Al, the frequencies differ with even slight variations in film thickness, electrode finger width, and electrode finger pitch of the IDTs. Thus, after the IDTs are formed, the frequencies are conditioned by trimming the IDTs. However, when an IDT is formed from Au so as to have H/λ of about 0.034, as an example, but the frequency is less than a desired value, such a frequency conditioning is carried out, causing the film thickness H/λ to be less than 0.033. That is, the propagation loss cannot be set at zero.